A real-time tissue blood flow monitor has been developed based on the velocity-dependent Doppler broadening of laser-light scattered by moving red blood cells within living tissues. A physical theory has been developed which explains the observed Doppler broadening of laser-light diffusing through tissue and formed the basis of a new instrument design. This instrument has demonstrated linear dependence on flow in a variety of animal and human tissues. The instrument monitors pulsatile flow with cardiac cycle as well as mean flow and has been used to characterize transient changes in the microvasculature. A clinical investigation of muscle blood flow at biopsy is being pursued in order to correlate the state of the microvasculature with various muscular diseases. Further development of the theory and instrumentation will be directed toward extending the clinical and research use of the technique at a variety of tissue sites.